Semiconductor light emitting diode structures have become the established leaders among optical light sources in different emission wavelength ranges. A conventional light-emitting structure is typically comprised of multiple layers of III-V compound semiconductor alloys such as GaAsP, AlGaAs, AlGaInP, or AlGaInN, depending upon the targeted wavelength emission of the diode structure. III-nitride AlGaInN alloys take special place among the possible material systems because of the wide range of available band-gaps. Light emission from AlGaInN covers the entire visible spectrum; III-nitride based light sources are currently being developed also for ultraviolet and infrared emission. Multiple active layer design of an optoelectronic device active region offsets a high level of optical and electrical losses and small strain relaxation length typical for III-nitride based heterostructures.
Multiple quantum well (MQW) design of the optically active region is beneficial for light emitter performance. By increasing the number of active quantum wells (QWs), the injected carriers can be spread among the MQWs thus decreasing the average QW population and minimizing the adverse effects of (i) nonradiative Auger recombination, (ii) QW thermal depopulation, and (iii) QW optical transition saturation. On the other hand, MQW active regions of electrically pumped devices typically suffer from inhomogeneous distribution of charge carriers, both electrons and holes, which are injected from the opposite sides of the diode structure. As a result, uneven and imbalanced population of active QWs unfavorably affects the device performance. In III-nitride light-emitting diodes (LEDs), the overpopulated active QWs often intensify the device efficiency droop either by increasing the nonradiative Auger recombination loss or by elevating the carrier leakage from the device active region. In laser diodes (LDs), the under-pumped QWs append their inter-band absorption to the total optical loss thus increasing the laser threshold.
In polar III-nitride heterostructures, non-uniform carrier injection is additionally aggravated by built-in polarization fields and related potential barriers. This sometimes makes nonpolar or semipolar technology an attractive alternative to polar templates. Nonpolar templates, however, do not solve the problem of inhomogeneous injection entirely. Even in the absence of internal polarization fields, MQW structures with sufficiently deep QWs and strong carrier confinement reveal uneven QW populations in a wide range of injection currents, so that the carrier population non-uniformity in III-nitride MQWs is a common feature of both polar and non-polar templates. Carrier injection inhomogeneity increases with active QW depth and, therefore, becomes more pronounced in the longer-wavelength emitters thus holding back the efficiency of III-nitride based light emitters in the so-called “green emission gap”.
Several conventional methods employ an MQW active region design in attempt to achieve multi-color emission with fixed or variable emission colors and/or to increase the injection efficiency of the device active region. For example, U.S. Pat. No. 7,323,721 describes a monolithic multi-color MQW structure designed to emit white light by including a sufficient number of QWs with different emission wavelengths, while U.S. Pat. No. 8,314,429 describes a multi-junction light emitting structure with the MQWs of each junction being designed to emit a specific wavelength that combines into white-light emission depending on the designed emission intensity of each of the multiple junctions comprising the structure. U.S. Pat. Nos. 7,058,105 and 6,434,178 describe approaches to achieve high carrier injection efficiency by incorporating means for increased optical and, respectively, electrical confinements of MQW active region. U.S. Patent Publication No. 2011/0188528 describes a MQW III-nitride light-emitting diode structure that achieves high carrier injection efficiency by using shallow QWs designed to avoid excessive carrier confinement and attain uniform MQW carrier population. U.S. Patent Publication No. 2010/0066921 describes a MQW III-nitride light emitting structure epitaxially grown on micro rods in which the epitaxial growth plane of the micro rods promotes higher indium incorporation in the semi-polar and non-polar orientation which can lead to multi-color emission form the MQW structure. Thus, the foregoing conventional approaches use particular ad hoc approaches relevant to their specific goals.